Interferometric modulator with dielectric layer

ABSTRACT

By selectively placing color filters with different transmittance spectrums on an array of modulator elements each having the same reflectance spectrum, a resultant reflectance spectrum for each modulator element and it&#39;s respective color filter is created. In one embodiment, the modulator elements in an array are manufactured by the same process so that each modulator element has a reflectance spectrum that includes multiple reflectivity lines. Color filters corresponding to multiple colors, such as red, green, and blue, for example, may be selectively associated with these modulator elements in order to filter out a desired wavelength range for each modulator element and provide a multiple color array. Because the modulator elements are manufactured by the same process, each of the modulator elements is substantially the same and common voltage levels may be used to activate and deactivate selected modulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/625,461, filed Nov. 24, 2009, which is a continuation of U.S. patentapplication Ser. No. 11/051,258, filed Feb. 4, 2005, which claimspriority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser.Nos. 60/613,542 and 60/613,491, both filed on Sep. 27, 2004, and U.S.Provisional Application Ser. No. 60/623,072, filed on Oct. 28, 2004. Theabove-identified applications are hereby expressly incorporated byreference in their entireties.

BACKGROUND

1. Field of the Invention

The field of the invention relates to microelectromechanical systems(MEMS), and, more particularly to interferometric modulators.

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. An interferometricmodulator may comprise a pair of conductive plates, one or both of whichmay be transparent and/or reflective in whole or part and capable ofrelative motion upon application of an appropriate electrical signal.One plate may comprise a stationary layer deposited on a substrate, theother plate may comprise a metallic membrane separated from thestationary layer by an air gap. Such devices have a wide range ofapplications, and it would be beneficial in the art to utilize and/ormodify the characteristics of these types of devices so that theirfeatures can be exploited in improving existing products and creatingnew products that have not yet been developed.

SUMMARY OF CERTAIN EMBODIMENTS

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

Certain embodiments of the invention provide a display device comprisingan array of spatial light modulators. Each spatial light modulator isindividually addressable so as to be switched between a first state inwhich the modulator is substantially reflective to at least onewavelength of light and a second state in which the modulator issubstantially non-reflective to the at least one wavelength of light.The display device further comprises an array of color filters. Eachcolor filter is positioned such that light reflected from acorresponding spatial light modulator propagates through the colorfilter. Each color filter substantially transmits the at least onewavelength of a corresponding spatial light modulator.

In certain embodiments, the spatial light modulator comprises aninterferometric modulator which comprises a fixed surface and a movablesurface substantially parallel to the fixed surface. In the first state,the movable surface is spaced a first distance from the fixed surface ina direction substantially perpendicular to the fixed surface. In thesecond state, the moveable surface is spaced a second distance,different from the first distance, from the fixed surface in a directionsubstantially perpendicular to the fixed surface. In certainembodiments, either the first distance or the second distance isapproximately zero. In certain embodiments, the first distance for eachof the spatial light modulators is approximately the same. In certainembodiments, the second distance for each of the spatial lightmodulators is approximately the same. In certain embodiments, the arrayof spatial light modulators comprises two or more subsets of spatiallight modulators, with the modulators of each subset each having thesame first distance and the same second distance.

In certain embodiments, the at least one wavelength of a spatial lightmodulator comprises a broadband wavelength region (e.g., white light).In certain embodiments, the at least one wavelength of a spatial lightmodulator comprises a narrowband wavelength region comprising two ormore colors. In certain embodiments, the at least one wavelength of aspatial light modulator comprises a single color of light (e.g., red,green, or blue light). In certain embodiments, the at least onewavelength comprises first-order light, while in other embodiments, theat least one wavelength comprises second-, third-, fourth-, orfifth-order light.

Other embodiments are possible. For example, in other embodiments, othertypes of light-modulating elements other than interferometric modulators(e.g., other types of MEMS or non-MEMS, reflective or non-reflectivestructures) may be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a released position and amovable reflective layer of a second interferometric modulator is in anactuated position.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIG. 6A is a cross section of the device of FIG. 1.

FIG. 6B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 6C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7 schematically illustrates an interferometric modulator arrayhaving three sets of modulator elements, each set having a correspondinggap distance.

FIG. 8 schematically illustrates one embodiment of an interferometricmodulator array in which substantially all of the modulator elementshave substantially equal gap distances.

FIG. 9 is a graph of an exemplary reflectance spectrum from aninterferometric modulator element having a gap distance d₀ approximatelyequal to one micron.

FIGS. 10A-10D are graphs of various reflectance spectra frominterferometric modulator elements compatible with embodiments describedherein.

FIGS. 11A and 11B schematically illustrate exemplary embodiments of adisplay device comprising an array of interferometric modulator elementsand an array of color filters.

FIG. 12 is a graph of transmittance spectra for a set of three exemplarycolor filter materials compatible with embodiments described herein.

FIGS. 13A-13D are graphs of the resultant reflectance spectra resultingfrom the combination of a color filter with the interferometricmodulator elements corresponding to FIGS. 10A-10D.

FIG. 14 schematically illustrates an interferometric modulator elementhaving a dielectric layer compatible with embodiments described herein.

FIG. 15 schematically illustrates another embodiment of a display devicewith an array of interferometric modulator elements compatible withembodiments described herein.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

By selectively placing color filters with different transmittancespectrums on an array of modulator elements each having the samereflectance spectrum, a resultant reflectance spectrum for eachmodulator element and it's respective color filter is created. In oneembodiment, the modulator elements in an array are manufactured by thesame process so that each modulator element has a reflectance spectrumthat includes multiple reflectivity lines. Color filters correspondingto multiple colors, such as red, green, and blue, for example, may beselectively associated with these modulator elements in order to filterout a desired wavelength range for each modulator element and provide amultiple color array. Because the modulator elements are manufactured bythe same process, each of the modulator elements is substantially thesame and common voltage levels may be used to activate and deactivateselected modulation.

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theinvention may be implemented in any device that is configured to displayan image, whether in motion (e.g., video) or stationary (e.g., stillimage), and whether textual or pictorial. More particularly, it iscontemplated that the invention may be implemented in or associated witha variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as thereleased state, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 13A isillustrated in a released position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 13A, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers are separated from the fixed metal layers by a defined airgap 19. A highly conductive and reflective material such as aluminum maybe used for the deformable layers, and these strips may form columnelectrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 13A,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application. FIG. 2is a system block diagram illustrating one embodiment of an electronicdevice that may incorporate aspects of the invention. In the exemplaryembodiment, the electronic device includes a processor 21 which may beany general purpose single- or multi-chip microprocessor such as an ARM,Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051,a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessorsuch as a digital signal processor, microcontroller, or a programmablegate array. As is conventional in the art, the processor 21 may beconfigured to execute one or more software modules. In addition toexecuting an operating system, the processor may be configured toexecute one or more software applications, including a web browser, atelephone application, an email program, or any other softwareapplication.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to a pixel array 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the released state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not releasecompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the released or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be released areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or releasedpre-existing state. Since each pixel of the interferometric modulator,whether in the actuated or released state, is essentially a capacitorformed by the fixed and moving reflective layers, this stable state canbe held at a voltage within the hysteresis window with almost no powerdissipation. Essentially no current flows into the pixel if the appliedpotential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Releasing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias).

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or released states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and releases the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thepresent invention.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 6A-6C illustrate three different embodiments of themoving mirror structure. FIG. 6A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 6B, the moveable reflective material 14is attached to supports at the corners only, on tethers 32. In FIG. 6C,the moveable reflective material 14 is suspended from a deformable layer34. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 34 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of well known techniques may be used toproduce the above described structures involving a series of materialdeposition, patterning, and etching steps.

Exemplary spatial light modulator arrays provide the capability toindividually address and switch selected modulator elements between atleast two states with different reflection and transmission properties.In certain embodiments, each spatial light modulator of the array can beoptimized to switch at least one corresponding wavelength from areflective “on” state to a non-reflective “off” state. The modulators ofsuch an array can be used in pixels of an electronic display device,either black-and-white or color.

In one embodiment, an interferometric modulator comprises a fixedsurface and a movable surface substantially parallel to the fixedsurface. In the reflective “on” state, the movable surface is spaced afirst distance from the fixed surface in a direction substantiallyperpendicular to the fixed surface. In the non-reflective “off” state,the moveable surface is spaced a second distance, different from thefirst distance, from the fixed surface in a direction substantiallyperpendicular to the fixed surface.

In one embodiment, the reflective “on” state of a black-and-whitedisplay reflects a plurality of wavelengths which sum to produce visiblewhite light, and the “off” state is substantially non-reflective for theplurality of wavelengths. For color displays, the reflective “on” statefor each modulator is reflective of one or more wavelengthscorresponding to a particular corresponding color (e.g., red, green, andblue).

In one embodiment, the color reflected by a modulator element in theactuated state is mainly determined by the optical path length of thedielectric layer, which is approximately the thickness of the dielectriclayer times the index of refraction of the dielectric material. Ingeneral, the thickness required for both the dielectric layer and theair gap to obtain the desired colors depends on the materials used inthe fixed and movable layers. Thus, the thicknesses of the dielectriclayer and air gap discussed herein with respect to certain embodimentsare exemplary. These thicknesses may vary depending on particularmaterials chosen for the dielectric and other characteristics of theparticular modulator elements. Accordingly, when different dielectricmaterials are used in modulator elements, the optical path distance maychange and the colors reflected by the modulator elements may alsochange. In one embodiment, the fixed layer of a modulator elementcomprises an Indium Tin Oxide transparent conductor layer, a Crpartially reflective layer, an Al reflective layer, and a dielectricstack comprising primarily SiO2.

For certain embodiments of the interferometric modulator arrays, a colordisplay is produced using three sets of modulator elements, each sethaving a different gap distance so as to switch a corresponding color.For example, as schematically illustrated by FIG. 7, an interferometricmodulator array 110 for use in a color display comprises a plurality ofmodulator elements, where each modulator element comprises a fixedsurface 112 and a movable surface 114. Between the fixed surface 112 andthe movable surface 114 a gap is defined, wherein a gap distance is thedistance between the fixed surface 112 and the movable surface 114. Theinterferometric modulator array 110 further comprises a planarizationlayer 116 which provides a planar surface for subsequent processing ofthe interferometric modulator array 110.

In the embodiment of FIG. 7, the modulator array comprises threemodulator element 120,122,124. Each of these modulator elements120,122,124 may be configured to reflect a different color so that thecombination of the three modulator elements 120,122,124 provides threecolors. For example, the modulator element 120 may be configured toreflect only a first color, the modulator element 122 may be configuredto reflect only a second color, and the modulator element 124 may beconfigured to reflect only a third color. In certain embodiments, thefirst, second, and third colors are red, green, and blue, while in otherembodiments, the first, second, and third colors are cyan, magenta, andyellow.

In the embodiment of FIG. 7, the first gap distance d₁ is set so thatthe first modulator element 120 is substantially reflective to a firstcolor (e.g., red), and non-reflective to a second and third color. Forthe second modulator element 122, the distance between the movablesurface 114 and the fixed surface 112 is selectively switched between asecond gap distance d₂ and approximately zero. In the embodiment of FIG.7, the second gap distance d₂ is set so that the second modulatorelement 122 is substantially reflective to a second color (e.g., green),and non-reflective to a first and third color. For the third modulatorelement 124, the distance between the movable surface 114 and the fixedsurface 112 is selectively switched between a third gap distance d₃ andapproximately zero. In the embodiment of FIG. 7, the third gap distanced₃ is set so that the third modulator element 124 is substantiallyreflective to a third color (e.g., blue), and non-reflective to a firstand second color.

As may be appreciated by those of skill in the art, fabrication of amulti-color modulator array, such as array 110, for example, typicallyinvolves use of three masks to pattern the sacrificial layers to producethe three different gap distances (corresponding to the three colors,e.g., red, green, and blue) between the fixed surface 112 and themovable surface 114 of the three modulator elements 120, 122, 124. Inaddition, building the mechanical structure of the modulator elementswith an uneven back structure increases the chances of misalignment andtilt of the modulator elements. In addition to the complexity offabricating modulator elements with three different gap distances,production of a deeply saturated color gamut (i.e., the set of possiblecolors within a color system) may be difficult. For example, a modulatorelement having a gap distance set to reflect red wavelengths of lightmay be fabricated using additional masking steps that increase the depthof color reflected by the modulator element. Thus, in some embodiments,the fabrication process includes production of a multi-color array ofmodulator elements with different gap distances and requires additionalsteps to enhance the color gamut of the array.

FIG. 8 schematically illustrates one embodiment of an interferometricmodulator array 1100 in which substantially all of the modulatorelements 1110 have substantially the same gap distance d₀. The gapdistance d₀ is selected to provide substantial reflectance by themodulator element 1110 to a selected range of wavelengths in the visiblelight portion of the spectrum. For example, in certain embodiments, thegap distance d₀ is approximately equal to one micron. The gap distanced₀ has been selected so as to produce a reflectance spectrum thatincludes multiple peaks.

FIG. 9 is a graph of an exemplary reflectance spectrum from a modulatorelement 1110 having a gap distance d₀ approximately equal to one micron.In this embodiment, the amount of light reflected from the modulatorelement 1110 is approximately 20-25% of the incoming light. In the graphof FIG. 9, the horizontal axis indicates the wavelengths of light thatare reflected from the exemplary modulator element 1110 and the verticalaxis indicates the percent reflectance from the exemplary modulatorelement 1110. As illustrated in the graph of FIG. 9, the reflectancespectrum of the modulator element 1110 includes three reflectivity peaksat about 430 nanometer, 525 nanometers, and 685 nanometers. Thus, themodulator element 1110 is said to have a reflectance spectrum includingthree reflectivity lines, or simply “lines,” where a line is a peak inreflectivity. In particular, the reflectance spectrum illustrated inFIG. 9 includes a first line 910, a second line 920, and a third line930. In other embodiments, the gap between the fixed and moveablesurface of the modulator element 1110 may be adjusted to produce more orless reflectivity lines. For example, in certain embodiments theselected range of wavelengths comprises a range of colors, thusproducing multiple reflectivity lines associated with the range ofcolors. In certain embodiments, the selected range of wavelengthscomprises two or more colors so that the reflectivity spectrum of themodulator element includes at least one reflectivity line associatedwith each of the two or more colors. In certain embodiments, theselected range of wavelengths comprises a selected color of light (e.g.,red, green, or blue light). In certain embodiments, the at least onewavelength comprises first-order light, while in other embodiments, theat least one wavelength comprises higher-order (e.g., second-, third-,fourth-, or fifth-order) light. In one embodiment, at the higher ordercolors, e.g., 6^(th) order, 3-6 reflectance peaks can appear in thevisible spectra simultaneously. FIGS. 10A-10D are graphs of exemplaryreflectance spectrums from modulator elements having varying gapsbetween their respective reflective and semi-reflective surfaces. FIGS.10A-10D each illustrate the reflectance (R), shown on the vertical axis,as a function of wavelength (λ), shown on the horizontal axis. Asindicated in FIG. 10A-10D, by adjusting the gap of the modulatorelement, the reflectance spectrum of the modulator element may beadjusted to include more than one line and the peak reflectivitywavelength of the one or more lines may also be adjusted.

The dashed lines in FIGS. 10A-10D denote a selected range of wavelengthsthat may be filtered by a color filter, for example. In certainembodiments, the selected range of wavelengths comprises a generallybroadband wavelength region (e.g., white light), as schematicallyillustrated by FIG. 10A. In certain embodiments, the selected range ofwavelengths comprises a broadband wavelength region with a single linepeaked at a selected wavelength (e.g., first-order red or first-ordergreen), as schematically illustrated by FIG. 10B. In certainembodiments, the selected range of wavelengths comprises a broadbandwavelength region comprising a plurality of lines corresponding todifferent colors, as schematically illustrated by FIG. 10C. In certainembodiments, the selected range of wavelengths comprises a wavelengthregion with a plurality of lines corresponding to colors of variousorders, as schematically illustrated by FIG. 10D. Other selected rangesof wavelengths are compatible with embodiments described herein.

FIGS. 11A and 11B schematically illustrate exemplary embodiments of adisplay device 1200 comprising an array of interferometric modulatorelements 1210 and an array of color filters 1220. FIG. 11A illustratesthree modulator elements 1210A, 1210B, and 1210C and three color filters1220A, 1220B, and 1220C. In the embodiment of FIGS. 11A and 11B, eachmodulator element 1210 is individually addressable so as to be switchedbetween a first state in which the modulator element 1210 issubstantially reflective to at least one wavelength and a second statein which the modulator element 1210 is substantially non-reflective tothe at least one wavelength. In the embodiment schematically illustratedby FIGS. 11A and 11B, each of the modulator elements 1210 has the samegap distance d₀ such that each modulator element 1210 switches the sameat least one wavelength as do the other modulator elements 1210.

Each color filter 1220 is positioned such that light reflected from acorresponding modulator element 1210 propagates through thecorresponding color filter 1220. In the embodiment schematicallyillustrated by FIG. 11A, the color filters 1220 are positioned outsidean outer surface 1230 of the array of interferometric modulator elements1210. In the embodiment schematically illustrated by FIG. 11B, the colorfilters 1220 are positioned within the outer surface 1230 and areintegral with the array of interferometric modulator elements 1210.

Each color filter 1220 has a characteristic transmittance spectrum inwhich a selected range of wavelengths is substantially transmittedthrough the color filter 1220 while other wavelengths are substantiallynot transmitted (e.g., either reflected or absorbed) by the color filter1220. In certain embodiments, the array of color filters 1220 comprisesthree subsets of the color filters 1220. Each color filter 1220 of thefirst subset has a first transmittance spectrum, each color filter 1220of the second subset has a second transmittance spectrum, and each colorfilter 1220 of the third subset has a third transmittance spectrum. Incertain embodiments, the first, second, and third subsets of the colorfilters 1220 have transmittance spectra corresponding to substantialtransmittance of red, green, and blue light, respectively. In certainother embodiments, the first, second, and third subsets of the colorfilters 1220 have transmittance spectra corresponding to substantialtransmittance of cyan, magenta, and yellow light, respectively.Accordingly, by placing the color filters 1220 with differenttransmittance spectrums on the modulator elements 1210, modulatorelements 1210 having the same gap distance may have differentreflectance spectrums. Thus, by combining color filters 1220corresponding to three colors (e.g., red/green/blue orcyan/magenta/yellow) with the modulator elements having substantiallyequal gap distances (e.g., the modulator elements schematicallyillustrated by FIGS. 8, 11A, and 11B), certain such embodimentsadvantageously provide reflectivity spectrums including three highlysaturated color lines without patterning the structure of theinterferometric modulator elements. In certain such embodiments, becausethe gap of each modulator element is substantially the same, commonvoltage levels may be used to activate and deactivate selected modulatorelements. Accordingly, voltage matching among the modulator elements issimplified.

In certain embodiments, color filters 1220 are combined with two or moresets of modulator elements having different gap distances (e.g., such asthe modulator elements schematically illustrated by FIG. 7), whereineach set of modulator elements reflects a different range ofwavelengths. In certain such embodiments, the color filters 1220 serveto tailor the reflectance spectra of the modulator element/color filtercombination (e.g., by removing unwanted tails or lines from theresultant reflectance spectrum). For example, in embodiments in which aset of modulator elements each has a reflective “on” state whichsubstantially reflects a range of wavelengths corresponding to red lightbut is substantially non-reflective of other wavelengths, a color filterhaving a transmittance spectra with a more narrow range of transmittedwavelengths of red light can result in a more deeply saturated red colorfrom the reflective “on” state of the modulator element. In certainembodiments, the color filter has a transmittance of less than 100% ofthe wavelengths which are substantially transmitted by the color filter.In certain such embodiments, the decrease in the overall displaybrightness due to the less-than-100% transmittance of the color filteris acceptable to generate the deeply saturated color.

FIG. 12 is a graph of transmittance spectra for a set of three exemplarycolor filter materials compatible with embodiments described herein. Theexemplary color filter materials of FIG. 12 are pigmented photosensitivecolor filter resins available from Brewer Science Specialty Materials ofRolla, Mo. The solid line of FIG. 12 corresponds to the transmissionspectrum of a 1.2-micron thick film of PSCBlue®, the dashed line of FIG.12 corresponds to the transmission spectrum of a 1.5-micron thick filmof PSCGreen®, and the dash-dot line of FIG. 12 corresponds to thetransmission spectrum of a 1.5-micron thick film of PSCRed®. Any type ofcolor filter know in the art, such as a pigment-based orinterference-based multilayer dielectric filter, for example, iscompatible with embodiments described herein.

The thicknesses of the color filter materials are selected to providethe desired transmission. When used with transmissive displays (e.g.,liquid-crystal displays) in which a backlight source is used to producelight which is transmitted through the display element, the lightpropagates through the color filter material only once. When used withreflective displays (e.g., reflective interferometric displays), thelight propagates through the color filter material twice: once whenincident on the modulator element and once when propagating away fromthe modulator element. Thus, the thickness of a color filter materialfor a reflective display is typically approximately one-half thethickness of the color filter material when used with a transmissivedisplay. Any type of color filter know in the art, such as apigment-based or interference-based multilayer dielectric filter, forexample, is compatible with embodiments described herein.

The dashed lines in FIGS. 10A-10D schematically illustrate a range ofwavelengths substantially transmitted by a selected color filter. FIGS.13A-13D are graphs of the reflectance spectra resulting from thecombination of this selected color filter with the modulator elements1210 corresponding to FIGS. 10A-10D. The resultant reflectance spectrumfrom the combination of the modulator elements 1210 corresponding to thereflectance spectrums illustrated in FIGS. 10A-10D and this selectedcolor filter corresponds to a convolution of the reflectance spectrum ofthe modulator elements 1210 and the transmittance spectrum of the colorfilter. The bandpass characteristic of the selected color filter allowsthe modulator elements 1210 to be used as separate color contributionsto the pixels of the display device.

With reference to FIGS. 11A and 11B, each of the modulator elements 1210may have a common gap that is sized so that the reflectance spectrum ofthe modulator elements 1210 includes three distinct reflectance lines,such as is illustrated in FIGS. 9 and 10D, for example. In oneembodiment, each of these three lines corresponds with red, green, orblue wavelengths. Accordingly, without the color filters 1220 themodulator elements 1210 would each have reflectance spectra includingthe three reflectance lines and the modulator elements 1210 would eachreflect white light when in an “on” state. However, with the addition ofthe color filters 1220, the modulator elements 1210 may be altered tovary their reflectance spectrums. For example, each of the color filters1220 may be selected to transmit only a certain range of wavelengths,such as red, green, or blue wavelengths. In particular, color filter1220A may be selected to transmit only a range of red wavelengths, colorfilter 1220B may be selected to transmit only a range of greenwavelengths, and color filter 1220A may be selected to transmit only arange of blue wavelengths. Accordingly, with the addition of the colorfilters 1220A-1220C, the modulator elements 1210 each provide differentreflectance spectrums. In particular, modulator element 1210A has asingle reflectance line at the range of blue selected by the colorfilter 1220A, modulator element 1210B has a single reflectance line atthe range of green selected by the color filter 1220B, and modulatorelement 1210C has a single reflectance line at the range of red selectedby the color filter 1220C.

In one embodiment, each modulator element includes a single color filterhaving a selected transmittance spectrum. In another embodiment,multiple modulator elements share a single color filter, such that theoutput of the multiple modulator elements are each filtered in the sameway. In another embodiment, a single modulator element includes multiplecolor filters.

FIG. 14 schematically illustrates an interferometric modulator element1300 compatible with embodiments described herein. In the embodiment ofFIG. 14, the modular element 1300 comprises a fixed layer 112 and amovable layer 114. In this embodiment, the fixed layer 112 includes areflecting surface on a layer that forms a partial reflector 1340. Adielectric layer 1310 is formed over this partial reflector 1340. In oneembodiment, the partial reflector 1340 comprises a thin layer ofchromium and the dielectric layer 1310 comprises silicon dioxide. Inother embodiments, the partial reflector 1340 and dielectric layer 1310may comprise any other suitable materials.

In certain embodiments, the materials and dimensions chosen for thedielectric layer 1310 vary the optical path length of the light withinthe modulator element 1300 and, accordingly, adjust the reflectancespectrum of the modulator element 1300. Various materials andthicknesses of the dielectric layer 1310 are compatible with embodimentsdescribed herein. As described in further detail below, an optical pathlength of the modulator element 1300 may be adjusted by changing thethickness of the air gap. Alternatively, the optical path length may bealtered by changing the thickness or material of the dielectric layer1310.

In one embodiment, the dielectric layer of the modulator element issized so that when the modulator is in the closed position, lightincident on the modulator element undergoes destructive interference anda viewer sees the modulator element as black. In such embodiments, thedielectric layer thickness may be about 300 to 700 Angstroms in order toprovide the proper destructive interference when the modulator elementis in the closed position.

In general, the power to switch a modulator element between two statesdepends in part on the capacitance between the electrically conductiveportions associated with the fixed and movable layers 112, 114. Thus, bydecreasing the gap distance, the capacitance between these surfaces isreduced, the switching power may also be reduced, and the total powerconsumption of a display comprising one or more modulator elements maybe reduced. In the embodiment of FIG. 14, the dielectric layer 1310 issized larger than 700 Angstroms so that the air gap may be decreasedwhile maintaining the desired optical path length for the modulatorelement to causes destructive interference of visible light when in theclosed state. Thus, with a smaller air gap, the power consumed by themodulator element may be decreased.

In the embodiment of FIG. 14, the dielectric layer 1310 has a thicknessof about 2200 to 2500 Angstroms, which may adjust the reflectancespectrum of the modulator element 1300 when in the closed state to be ina range of wavelengths between first-order red light and second-orderblue light. This range of wavelengths is not a true black, because itincludes the tails of the first-order red light and the second-orderblue light, resulting in a “deep purple” color. This deep purple maysufficiently resemble black to be used as a black state of a pixel.However, in certain embodiments, as schematically illustrated by FIG.14, the modulator element 1300 includes a color filter 1320 having atransmittance spectrum that does not transmit the tails of thefirst-order red light and the second-order blue light. Such embodimentsprovide a non-reflective closed state of the modulator element 1300which more closely approximates true black. The color filter 1320 mayfurther be selected to transmit only a selected wavelength range whenthe modulator element 1300 is in the open state. The modulator element1300 may also provide lower capacitance, and thus consume less power,than a similar modulator element 1300 having a thinner dielectric.

FIG. 15 schematically illustrates a portion of another embodiment of adisplay device 1400 including an array of interferometric modulatorelements 1410 compatible with embodiments described herein. In thisembodiment, the gap distance when the modulator element is in thereflective “on” state is less than the gap distance when the modulatorelement is in the non-reflective “off” state. The modulator element 1400includes a dielectric layer that is thin enough to frustrateinterference effects between the partially reflective and fullyreflective layers and to therefore reflect substantially all wavelengthsof light with equal intensity, when the modulator element is in the “on”state. In one embodiment, the dielectric thickness is about 100Angstroms. In another embodiment, the dielectric thickness is in therange of about 50 to 200 Angstroms.

In one embodiment, a gap distance d₀ is set sufficiently small so thatin the reflective “on” state the modulator element 1400 providesapproximately 100% reflectance of visible light, which may besignificantly more reflectance than from embodiments with larger gapdistances. Accordingly, certain embodiments of the display device 1400may provide a black-and-white display with improved reflectance. Colorfilters 1420 may be used to tune the color spectrum of the modulatorelements 1410 in the same manner as described above.

In the embodiment of FIG. 15, the gap distance in the non-reflective“off” state is larger than d₀ and is selected to not reflect a broadrange of wavelengths. In particular, the gap distance is such that lightundergoes destructive interference between the fixed and movablesurfaces of the modulator elements 1410, causing substantially no lightto reflect from the modulator element 1410 when in the “off” state. Inone embodiment, the gap distance in the “off” state is in the range ofabout 500 to 1200 Angstrom.

Certain embodiments described herein advantageously provide highlysaturated colors using a single gap distance for substantially all ofthe modulator elements of the interferometric modulator array. Certainembodiments described herein advantageously do not require specialpatterning or masking of the reflective layer in modulator elementsconfigured to have reflectivity lines in the red wavelengths. Certainembodiments advantageously provide a sufficiently large gap distance tobe tuned to eliminate unwanted portions of the visible spectrum. Certainembodiments advantageously provide a sufficiently small dielectricthickness to reflect approximately 100% of a broad range of visiblewavelengths. Certain embodiments advantageously provide alow-capacitance interferometric modulator structure.

Various embodiments of the invention have been described above; however,other embodiments are possible. For example, in other embodiments, othertypes of light-modulating elements other than interferometric modulators(e.g., other types of MEMS or non-MEMS, reflective or non-reflectivestructures) may be used.

Accordingly, although this invention has been described with referenceto specific embodiments, the descriptions are intended to beillustrative of the invention and are not intended to be limiting.Various modifications and applications may occur to those skilled in theart without departing from the true spirit and scope of the invention.

What is claimed is:
 1. An interferometric modulator comprising: apartially reflective surface; a reflective surface, wherein at least oneof the partially reflective surface and the reflective surface ismovable with respect to the other to provide open and closed states; anda dielectric layer between the partially reflective surface and thereflective surface, wherein the dielectric layer has a thicknessconfigured to cause the interferometric modulator to produce areflectance spectrum that includes a tail of second-order blue lightwhen in the closed state.
 2. The interferometric modulator of claim 1,wherein the dielectric layer has a thickness of at least about 700Angstroms.
 3. The interferometric modulator of claim 1, wherein thedielectric layer has a thickness in the range of about 2200 to 2500Angstroms.
 4. The interferometric modulator of claim 1, wherein thedielectric layer includes an oxide film.
 5. The interferometricmodulator of claim 1, wherein the closed state produces a dark state andthe open state produces a bright state.
 6. The interferometric modulatorof claim 1, further comprising a filter structure configured toattenuate the tail of the second-order blue light.
 7. Theinterferometric modulator of claim 1, wherein the reflectance spectrumfurther includes a tail of first-order red light.
 8. The interferometricmodulator of claim 7, further comprising a filter structure configuredto attenuate the tail of the first-order red light and the tail of thesecond-order blue light to darken the dark state of the interferometricmodulator.
 9. The interferometric modulator of claim 1, wherein thepartially reflective surface is fixed and the reflective surface ismovable.
 10. An apparatus comprising a display including a plurality ofinterferometric modulators according to claim
 1. 11. The apparatus ofclaim 10, further comprising: a processor that is configured tocommunicate with the display, the processor being configured to processimage data; and a memory device that is configured to communicate withthe processor.
 12. The apparatus as recited in claim 11, furthercomprising a driver circuit configured to send at least one signal tothe display.
 13. The apparatus as recited in claim 12, furthercomprising a controller configured to send at least a portion of theimage data to the driver circuit.
 14. The apparatus as recited in claim11, further comprising an image source module configured to send theimage data to the processor.
 15. The apparatus as recited in claim 14,wherein the image source module includes at least one of a receiver,transceiver, and transmitter.
 16. The apparatus as recited in claim 11,further comprising an input device configured to receive input data andto communicate the input data to the processor.
 17. An interferometricmodulator comprising: a partially reflective surface; a reflectivesurface, wherein at least one of the partially reflective surface andthe reflective surface is movable with respect to the other to provideopen and closed states; and means for electrically insulating disposedbetween the partially reflective surface and the reflective surface,wherein the electrically insulating means has a thickness configured tocause the interferometric modulator to produce a reflectance spectrumthat includes a tail of second-order blue light when in the closedstate.
 18. The interferometric modulator of claim 17, wherein theelectrically insulating means includes a dielectric layer.
 19. Theinterferometric modulator of claim 17, wherein the electricallyinsulating means has a thickness of at least about 700 Angstroms. 20.The interferometric modulator of claim 17, wherein the electricallyinsulating means has a thickness in the range of about 2200 to 2500Angstroms.
 21. The interferometric modulator of claim 17, wherein theelectrically insulating means includes an oxide film.
 22. A method ofmanufacturing an interferometric modulator, the method comprising:fabricating a partially reflective surface; fabricating a reflectivesurface, wherein at least one of the partially reflective surface andthe reflective surface is movable with respect to the other to provideopen and closed states; and positioning a dielectric layer between thepartially reflective surface and the reflective surface, wherein thedielectric layer has a thickness configured to cause the interferometricmodulator to produce a reflectance spectrum that includes a tail ofsecond-order blue light when in the closed state.
 23. The method ofclaim 22, wherein the dielectric layer has a thickness of at least about700 Angstroms.
 24. The method of claim 22, wherein the dielectric layerhas a thickness in the range of about 2200 to 2500 Angstroms.
 25. Themethod of claim 22, wherein the dielectric layer includes an oxide film.26. A method of operating a display, the method comprising: receivinglight from a light source so that the light at least partially passesthrough a partially reflective surface and reflects from a reflectivesurface, wherein an optical cavity is formed between the partiallyreflective surface and the reflective surface; and actuating at leastone of the partially reflective surface and the reflective surface to aclosed state, wherein a dielectric layer is positioned between thepartially reflective surface and the reflective surface, the dielectriclayer having a thickness configured to cause the closed state to producea reflectance spectrum that includes a tail of second-order blue light.27. The method of claim 26, wherein the dielectric layer has a thicknessof at least about 700 Angstroms.
 28. The method of claim 26, wherein thedielectric layer has a thickness in the range of about 2200 to 2500Angstroms.
 29. The method of claim 26, further comprising attenuatingthe tail of the second-order blue light.